An Etymological Dictionary of the German Language/Annotated/Füllen
Füllen, neuter, from the equivalent Middle High German vülîn, Old High German fulîn, neuter, besides Middle High German vüle, Old High German fulî, neuter, ‘foal’; for the affix -în-, denoting the young of animals, see under Schwein. Based upon Fohlen (Goth fula); hence *ful-ein, neuter has to be assumed in Gothic; compare Middle Low German völen, Dutch veulen, Old Icelandic fyl. Another derivative of ful- is Old High German fulihha, Middle High German vülhe, feminine, ‘filly,’ pointing to Gothic *fuliki.